1. Field of the Invention
The present invention relates to a distance measuring system and method for measuring the distance between two points utilizing an ultrasonic wave.
2. Description of the Related Art
As a system for measuring the distance between two points, Japanese Patent Laid-Open No. 2006-64504 (hereinafter called “Patent Document 1”) describes an ultrasonic distance measuring apparatus for measuring the distance between a transmitter and a receiver based on a difference between arrival times at the receiver of an infrared radiation and an ultrasonic wave which have been simultaneously emitted from the transmitter.
The ultrasonic distance measuring apparatus described in Patent Document 1 calculates the distance based on the time required for the ultrasonic wave, which travels at the velocity of sound, to be transmitted from the transmitter to the receiver on the assumption that infrared radiation is emitted and received at substantially the same time. The receiver rectifies an ultrasonic wave reception signal using a half-wave rectifier circuit, the output voltage of which is smoothed by a capacitor. Then, the receiver determines that the ultrasonic wave is received when the output voltage of the capacitor exceeds a previously set threshold. The receiver determines a period from the time the infrared radiation reception signal was detected to the time the output voltage of the capacitor exceeds the threshold voltage, as the time required for the ultrasonic wave to be transmitted from the transmitter to the receiver.
However, the conventional distance measuring system as described above can erroneously determine the distance between the transmitter and receiver if an obstacle exists outside of a measurable area (area in which the system can measure the distance between the transmitter and receiver), and if, other than an ultrasonic wave which arrives directly at the receiver from the transmitter (hereinafter called the “direct wave”), as illustrated in FIG. 1, another ultrasonic wave from the transmitter arrives at the receiver after it has been reflected by the obstacle (hereinafter called the “reflected wave”).
For example, when the distance between the transmitter and receiver is measured at each predetermined time to detect movements of the transmitter (or receiver), the transmitter repeatedly emits the infrared radiation and ultrasonic wave at constant period T. In this event, if a reflected wave of an ultrasonic wave emitted from the transmitter at certain period T is detected between an infrared radiation reception signal at the next period and a received direct wave signal, and if the received reflected wave signal exceeds the threshold voltage, as illustrated in FIG. 2, the receiver calculates the distance from arrival time tb of the reflected wave, rather than from arrival time ta of the direct wave. This causes a problem of reduced measurement accuracy of the distance between the transmitter and receiver.